At present, the existing method for gasifying coal or oil shale subsurface is to execute the steps of drilling wells, penetrating, igniting, blasting air and drawing the product air. The disadvantage of the said method is that the obtained fuel gas has a relatively low caloric value, since a large amount of stabilizing gas that is generated when organic substances combusts in the underground gasification region blends with fuel gas. That method has been disclosed in the Russian patent NO. 2385412, with the Patent Classification No. MΠ E21MB43/295.
Another known method is as follows: drilling at least one well until its depth penetrates through the seam working region; creating at least one crack penetrating through the well; filling the crack with an electrically-conductive material and inserting two electrodes therein; making the electrodes in contact with the electrically-conductive material; electrifying the electrodes so that electric current in the crack flows partially or entirely along the electrically-conductive material; and thereby by using the resistance heat-releasing function of the electrically-conductive material, releasing sufficient heat for effecting thermolysis of given organic substances in the seam. The disadvantage of the said method lies in the complex process and great construction intensity, and further the conductive material may be toxic, thereby polluting the environment and underground water. That method has been disclosed in the Russian patent NO. 2349745, with the Patent Classification No. MΠ E21B43/24.
The U.S. Pat. No. 4,084,638 discloses a method of production stimulation and enhanced recovery of oil. The method comprises drilling at least two wells downwardly from the ground surface, putting electrodes into the wells respectively, conducting high voltage into the electrodes to crush the oil shale, and heating the oil shale by means of the resistance thermal effect of the seam. In the method, the high voltage only achieves the shocking function to create cracks in the oil shale, thereby forming passages in the oil shale for facilitating flow of the oil and gas. During this process, the oil shale appearance undergoes a change, a kind of simple physical change. In addition, during the process of executing the method, shocking is independent of electrical heating, and these two steps will not have an effect on each other. The method mainly has the following technical effects: creating the cracks by means of shocking to improve the flow passage of the oil and gas, improving flowability of the oil and gas via heating, and thereby increasing the recovery rate of the oil and gas. The method also can be described as a means for improving exploitation of the presently existing oil and gas reservoir.
Shell Group of Companies developed electric heating technology, ICP technology for short, and Mahogany research engineering of Shell has been always dedicated to renovating the Shell conversion technology in-situ. Shell filed the patent application “Method for Producing Oil by Heating Oil Shale” on Jan. 17, 1987, with the filing No. 87100890 and publication No. CN87100890A. The disclosed principle is as follows: inserting the electrical heaters into the well, and normally heating the oil shale away from the ground surface by 300-600 m. The rock formation is slowly heated to 400-500° C., and kerogen in the oil shale is converted into crude oil and rock gas. Then the products (crude oil and rock gas) are suctioned upwardly onto the ground by using the conventional exploiting method.
In comparison to the conventional ground dry distillation treatment, the slow heating at the relatively low temperature in-situ obviously reduces the carbon emissions. The ICP process of Shell brings about approximately ⅓ rock gas and ⅔ light crude oil, and the produced rock gas is subjected to power generation or sale. Shell Group of Companies has proved that the ICP process requires less than three barrels of water for producing one barrel of crude oil or equivalent.
Exxon Mobil Corporation filed the patent application No. 200880009037.3 on Mar. 7, 2008 entitled “Resistive Heater for in situ Formation Heating”, with the publication No. CN10163655A. The disclosed technology is as follows: fracturing oil shale by hydraulic power, pouring a conductive material into the crack to form a heating portion, and heating oil shale in-situ by means of dielectric resistance. Here is the principle: creating vertical crack in the horizontal well, filling the crack with conductive media to obtain a conductive region, heating shale oil to the thermolysis temperature through the conductive region, and producing crude oil and rock gas that can be withdrawn by the conventional oil production technology.
Exxon Mobil Corporation filed the patent application No. 200780046031.9 on Oct. 10, 2007 entitled “Enhanced Shale Oil Production by in situ Heating using Hydraulically Fractured Producing Wells”, with the publication No. CN101558216A. The fracturing technology was previously regarded as the most attractive technique among more than thirty standby solutions by the Exxon Mobil Corporation, and linear thermal conduction in the planar heat reservoir of the heating body possibly is the most effective approach for entrance of ore-rich layer and for achieving conversion to the crude oil and rock gas. According to the experiences offered by Exxon, the planar heater needs less heating wells and occupies less area in comparison to the wellhole heater. The in-situ technique owned by Exxon Mobil Corporation possibly also needs to take measures to avoid intrusion of formation water, and to prevent formation water from being polluted by the formed carbohydrates and other components. In this regard, a great deal of power is needed for use in heating.
Radiation heating technique: RF/CF technique of Raytheon Corporation.
Such in-situ technology employs radio-frequency and injection of supercritical carbon dioxide for heating oil shale to thermolysis temperature, so as to drive the liquid and gas into the production well. On the ground surface, the carbon dioxide fluid is separated and again injected into the production well, and at the same time the oil and gas are refined into gasoline, fuel oil and other products. Such extracting technology just requires several months for producing oil and gas, while several years by other in-situ methods. The said technology can adjust the heat energy applied to the object layer to produce the various products. Like the ICP process of Shell, the RF/CF technique also consumes a great deal of power for forming the radio-frequency energy. According to the experiences offered by Raytheon Corporation, use of such technique can produce 4-5 barrels of crude oil or equivalent by consuming the energy of one barrel of crude oil.